Lewis & Short

Pŏsīdōnĭus, ii, m., = Ποσειδώνιος, a celebrated Stoic at Rhodes, a disciple of Panœtius and instructor of Cicero, Cic. Att. 2, 1, 2; id. Off. 3, 2, 8; id. N. D. 1, 3, 6; id. Div. 1, 3, 6 al.